1. Field of the Invention
The present invention relates to a process for the preparation of water-soluble acrylic polymers by photopolymerization.
2. Description of the Prior Art
The polymerization of acrylic monomers in dilute aqueous solutions to obtain polymers in the form of gels has long been known. However, such processes require the product gel to be dried and ground which is economically disadvantageous, since the dehydration of a gel is a long and costly operation.
In the particular case of polymers of high molecular weight and high water-solubility, initially obtained from a relatively concentrated solution of monomers, the viscosity continuously increases during polymerization, and it is very difficult to achieve good and efficient stirring of the reaction mixture and suitable dissipation of the heat of reaction. Tremendous difficulties too have been encountered with a polymerization solution having a percentage of dissolved monomers of about 20%.
In French Pat. No. 1,518,053, filed Feb. 21, 1967, a polymerization method employing an aqueous solution containing 30 to 50% of monomers is described, in which a heat-dissipating container is used as the polymerization vessel. This method eliminates the need to stir the reaction mixture, but requires a certain cooling device to remove the heat generated. The polymerization time is quite long and the rubbery mass obtained must be subjected to the aforenoted drying/grinding operations.
French Pat. No. 2,064,729 describes a process for obtaining solid, high molecular weight water-soluble acrylic polymers or copolymers utilizing either a continuous or discontinuous process beginning with a concentrated aqueous solution of monomers preferably having a pH between 8 and 13. The process consists of adding to the initial solution of monomers a photoinitiator, preferably an alkyl of benzoin, in quantities between 0.05 and 10% by weight relative to the monomers, depositing the solution as a thin layer or droplets on a fixed or movable waterproof support, and, finally, subjecting the coated support to irradiation with light for 30 seconds to 15 minutes. This process affords acrylic polymers and copolymers having very valuable properties, particularly in the field of flocculating agents, due to their high molecular weight and high water-solubility.
Nevertheless, until the present invention it has not been possible to obtain high molecular weight polymers devoid of insoluble fractions and/or unpolymerized monomer.
Thus, there remains a need in the art for an economically advantageous means of preparing high molecular weight, water-soluble acrylic polymers lacking insoluble factions and unpolymerized monomer. This need is especially acute in the field of water purification and treatment where regulations exist which limit the quantity of residual monomer which may be present in the polymer system; for example, the F.D.A. has established 0.05% as the maximum residual monomer concentration.